1. Field of the Invention
This invention relates to a tapping machine which executes a tapping operation by operating a spindle motor and a feed axis motor in synchronization.
2. Description of the Related Art
Generally, a tapping machine, includes a tapping tool and a spindle head that is equipped with the tapping tool and moves up and down. The tapping tool rotates about a spindle being driven by a spindle motor. As a feed axis is driven by a feed axis motor, the spindle head moves up and down along the feed axis.
Japanese Registered Patent No. 2629729 discloses a feed axis that moves following a spindle, and a feed axis instruction and a correction value are produced based on the position of the spindle and the pitch of a screw. Japanese Examined Patent Publication No. 3-43012 discloses a system which executes tapping through an interpolation circuit (in synchronization with an instruction). Further, Japanese Unexamined Patent Publication No. 2003-181722 discloses a synchronization error that is stored, and the position instruction data is corrected by using the stored synchronization error.
Japanese Registered Patent No. 3097181 discloses calculating feed accelerations and total acceleration from the rotational position of the spindle, feed deviation thereof and from the feed deviation of the feed axis motor, produces a rotation instruction value based thereupon and upon the pitch of the screw, produces a rotation correction value from the feed position based on the pitch of the screw, and drives a spindle motor based on the rotation instruction value corrected by the rotation correction value.
However, in the case that the tapping operation of the system in which the feed axis follows the spindle, precision in the synchronization varies depending upon the response of the feed axis. There is no problem if the gain of the feed axis can be set to be very large. However, there are some cases in which it is not often allowed to set the gain of the feed axis to be so large. In such a case, response of the feed axis delays at the time when the spindle rotates reverse and, as a result, synchronization error increases. Further, none of the Japanese Registered Patent No. 2629729, Japanese Examined Patent Publication No. 3-43012, Japanese Unexamined Patent Publication No. 2003-181722 or Japanese Registered Patent No. 3097181 discloses about decreasing the synchronization error when the spindle is reversely rotated.
The present invention was accomplished in view of the above circumstances, and has an object of providing a tapping machine that is capable of decreasing a synchronization error between the spindle and the feed axis even when it is not allowed to set the gain of the feed axis to be so large.